1. Field of Invention
The invention relates generally to acoustic sensors and more particularly to fiber-optic acoustic sensors.
2. Brief Description of Related Art
The use of laser interferometers for detection of acoustic signals in a fluid medium is known. Typical prior art devices have been configured by winding an optical fiber onto a pressure sensitive mandrill or by suspending a pressure sensitive fiber in an acoustic medium, typically in a spiral configuration.
The limitations of prior art devices typically used in a moving flow field include the requirement of suspending the array in a manner that does not restrict the acoustic response. This requirement greatly complicates suspension. Prior sensor arrays are typically configured either three-dimensionally or in planar arrays. Planar sensor arrays, while more easily suspended by attachment to a hull or other mounting surface, have limited directional characteristics thus reducing omni-directional operation. The three-dimensional configuration, on the other hand, while exhibiting superior omni-directional capabilities, have increased suspension difficulties thus limiting use in a moving flow field.
In the low wave number domain experienced by hull structures, pressure fluctuations in the turbulent boundary layer induce vibration in the hull structure resulting in vibrational noise. The wavenumber characteristics of the turbulent boundary layer pressure fluctuations have large values at the structural wavenumber response of the hull of underwater vehicles. The hull structure in these circumstances is excited by the boundary layer pressure fluctuations and the resulting vibrations become a significant noise source. Prior sensors are sensitive to this vibration induced noise and, due to the necessity of suppressing vibrational noise, are limited in sensitivity to incident acoustic energy signals. The vibration induced noise and impinging acoustic signals are in the same frequency range and therefore cannot be filtered by conventional means. Therefore, the presence of vibrational noise greatly reduces the effectiveness of prior hydrophone sensors in detecting and isolating acoustic energy signals from the fluid medium. In the case of the omni-directional configuration, the directional characteristics of the array limit the omni-directional sensitivity.